The soldering of solder bumps, which are protruding solder electrodes provided on electronic components, onto their respective electrodes on a workpiece is commonly employed as a method for mounting electronic components. One common method for forming a solder bump is to place a solder ball on the electrode of an electronic component, and then to melt the solder ball by heating to solder it onto the receiving electrode.
In some types of electronic components, however, a solder bump is formed on a cavity provided on an insulation layer such as TAB (tape-automated bonding) tape overlaying the electrode, creating an electrical contact between the solder bump and the electrode at the bottom of the cavity. With the mounting density of electronic components continuing to increase, the alignment pitch of solder bumps is contracting. The diameter of the cavities is thus decreasing.
However, a solder ball for forming a solder bump needs to be larger than a certain size to secure the required solder volume. Accordingly, the diameter of the cavity is often smaller than the diameter of its corresponding solder ball for forming the solder bump. Since the solder ball cannot go through the opening of the cavity, the solder ball mounted on the cavity is caught by the opening. This may create a gap between the bottom face of the solder ball mounted on the cavity and the surface of the electrode at the bottom of the cavity.
When the solder ball is heated in the reflow process in this condition, the solder ball melts without contacting the electrode, and thus the solder ball and the electrode are not actually soldered. To prevent this defect, solder paste is conventionally provided inside the cavity. Solder paste contains solder particles suspended in flux. This conventional method aims to improve the solderability of the solder ball and electrode by providing solder particles in the cavity.
However, the solder particles in the solder paste employed in the conventional solder bump formation method have the same liquidus temperature as the solder ball, and thus, in the reflow process, the solder particles melts at the same time as the solder ball. The problem is that the larger volume of melted solder particles in the cavity is absorbed by the melted solder ball formed of melted solder ball. In other words, a large percentage of the solder particles provided in the cavity with the intention of providing an electrical connection between the solder ball and the electrode coalesced with the solder ball and is merely increasing the volume of the solder ball and failing to produce the intended effect.
Accordingly, the melted solder, which is a melted solder ball, still fails to contact the electrode in the cavity, and subsequently solidifies without contacting the electrode at the bottom of the cavity. In particular, if a solder bump is formed in a cavity having an opening shape through which the solder ball cannot go, defective solder bump formation often occurs even if solder paste is provided in the cavity. The same problem also occurs when mounting spherical solder bumps on the circuit electrodes of a substrate. More specifically, the same problem occurs when a solder bump is soldered onto the electrode at the bottom of a cavity on the substrate which has an opening shape through which the solder bump cannot go.
The present invention provides a solder bump formation method and solder bump mounting method which prevent defective soldering.